Japanese Post-examination Publication No. 40-3353 of a utility model application discloses a water heater which is supplied with electric power from the primary of a transformer and uses the secondary coil as a tubular conductive heating element to heat water flowing therein. The heating water-inlet tube, i.e. the secondary coil, as well as the primary coil (wire coil) of the transformer is insulated throughout its entire length and entire surface area. That is, multiple turns of the secondary coil are insulated from each other to prevent short-circuit between respective turns thereof. Opposite ends of the water inlet tube, i.e. the entrance and exit of water, are electrically connected to prevent electrical leakage to the exterior of the water inlet tube.
This system certainly operates well when it heats water up to a modest temperature about 100.degree. C. in the standard atmospheric pressure. However, if it is used as a large-scaled heater subject to a high temperature, high voltage and high pressure, various difficulties arise in its mechanical structure, and the efficiency of the transformer decreases. Further, this system fails to effectively use heat of the primary coil.